A headphone having a noise-cancelling function (noise-cancelling headphone) includes a microphone and a noise-cancelling circuit (hereinafter referred to as “NC circuit”). The microphone collects external sounds (hereinafter referred to as “noises”) around the headphone. The NC circuit generates cancelling signals corresponding to the noises collected by the microphone. The headphone combines sound waves corresponding to the cancelling signals generated by the NC circuit and sound waves corresponding to the reproduced signals from a sound source such as a music player connected to the headphone, and then outputs the combined sound waves from a driver unit. That is, the headphone outputs musical sounds (hereinafter referred to as “reproduced sounds”) derived from the reproduced signals from the sound source with cancelling (muting) the noises.
Examples of the scheme for generating cancelling signals include a feedback scheme (hereinafter referred to as FB) and a feedforward scheme (hereinafter referred to as FF).
A built-in microphone of the FB headphone is disposed in the interior of a housing unit (ear piece) of the headphone and near the ear of the user. The NC circuit analyses in real-time the signals of the noises collected by the microphone and generates cancelling signals minimizing the noises at the position of the eardrum of the user. The FB headphone collects the noises at the position near the ear of the user. Thus, the noise-cancelling effect of the FB headphone is higher than that of the FF headphone. In addition, the FB headphone can respond to variable noise components.
However, when the built-in microphone of the FB headphone collects the reproduced sounds in addition to the noises and then generates cancelling signals, the sound quality of the reproduced sounds output from the headphone is degraded. In addition, the ear piece of the FB headphone should be sealed in the state which the headphone is worn on the head of the user, to enhance the noise-cancelling effect. When the ear piece is sealed, the reproduced sounds output from the headphone may be muffled. Thus, the sound quality of the reproduced sounds is degraded. To address the problem, a FB headphone generally includes a filter to correct the sound quality of the reproduced sounds.
On the other hand, a built-in microphone of the FF headphone is disposed at the exterior of the housing unit of the headphone. The NC circuit analyses the signals of the noises collected by the microphone and predicts the variation of the noises that will reach the eardrum of the user wearing the headphone. The NC circuit generates cancelling signals based on the result of the prediction. The FF headphone does not need the placement of the microphone on a limited space near the ear of the user. Thus, the FF headphone is suitable for downsizing headphones. In addition, the built-in microphone of the FF headphone is disposed at a position remote from the driver unit. Thus, the sound quality of the reproduced sounds output from the FF headphone is less susceptible to the noise than the sound quality of the reproduced sounds output from the FB headphone.
However, the FF headphone readily indicates directionality of the noise-cancelling effect depending on the position of the built-in microphone, and is susceptible to the influence by the blowing of the wind, for example. As a result, the FF headphone can cause a feeling of strangeness or discomfort to the user, when the noise-cancelling function is activated.
Regardless of the presence of the noise-cancelling function, the sound quality of the sound wave (sound) output from the driver unit of the headphone is affected by the air pressure in an air chamber disposed behind the driver unit. Thus, many headphones have vent holes to adjust the air pressure in the air chamber disposed behind the driver unit. The vent holes for adjusting the sound quality establish the communication between the air chamber disposed behind the driver unit and the exterior of the housing unit of the headphone.
Schemes have been proposed to provide vent holes for adjusting the sound quality to noise-cancelling headphones (for example, refer to U.S. Pat. No. 6,735,316). The noise-cancelling headphone disclosed in U.S. Pat. No. 6,735,316 is the FB noise-cancelling headphone.